Optical communications systems are well known in the art. A typical optical communication system may include a laser transmitter that converts an electrical signal into a modulated optical signal. The optical signal is carried over the optical-fiber link and converted back to an electrical signal by a photodetector in the optical receiver. Modulation of the optical signal may be accomplished by directly modulating the intensity of the laser via modulating the bias current in response to the electrical signal. This technique is referred to as direct modulation. A second method in common use for modulating the optical intensity is referred to as external modulation. With external modulation, the source laser is operated in the CW mode and its output is connected to an electro-optic modulator in which the optical intensity is modulated by the electrical signal. Both types of optically modulated systems are in common use in CATV systems.
CATV optical fiber transmission systems generally carry a large number of frequency-division multiplexed (FDM) analog and digital signals. One of the problems encountered by such systems is the need for linearity within the system. For example, such systems typically require a high degree of linearity in the electrical-to-optical modulation process and a high degree of linearity in the fiber link and optical receiver. Nonlinearity in a transmission results in undesirable impairments of the received television picture or loss of data in a digital application. For example, in CATV systems, composite second-order (CSO) distortion refers to the total distortion power in a channel due to second-order intermodulation of the radio frequency (RF) carriers. Distortion occurs at frequencies equal to the sum frequencies and the difference frequencies of the interfering carriers. Composite triple-beat (CTB), or third-order distortion, is the total distortion power in a channel due to third-order intermodulation of the RF carriers. In addition to the second-order and third-order distortions, higher-order distortions may occur to a lesser degree in transmissions.
One of the primary sources of nonlinearity or distortion is the electrical-to-optical converter. For directly modulated optical transmitters, the laser diode normally limits the achievable optical transmission performance. It is generally known that the laser diode, such as a distributed-feedback laser, typically introduces an amount of unwanted distortion to the optical signal. A laser diode produces distortion from several causes. A laser generally exhibits a static nonlinearity, which is evident in the nonlinearity of the laser light intensity characteristic as a function of electrical bias current. Static distortion is a function only of the instantaneous amplitude of the input to the laser, and is not frequency dependent.
Lasers also generate distortion from dynamic causes. These dynamic distortions are dependent not only on the distortion frequency, but also on the frequencies of the carriers that cause the distortion. In a nonlinear device, such as a distributed-feedback laser, the amplitude and phase of the distortion of each beat is a function of the amplitude and phase of each carrier that produced the distortion. In summary, nonlinear devices (such as lasers) inherently and undesirably generate distortion.
It is well known to use predistortion and post distortion techniques to cancel the inherent distortion generated in a nonlinear device. With predistortion, the electrical signal to be transmitted is fed to an ancillary circuit which generates distortion equal in magnitude to the distortion inherent in the nonlinear device, but of the opposite sign. In this manner, when the electrical signal and the generated distortion (called predistortion) pass through the nonlinear device, the generated distortion operates to cancel the inherent nonlinear distortion of the device due to the phase relationships and relative amplitudes of the two distortion components.
In general, predistortion circuits can be classified as having a parallel-branch circuit topology or an inline circuit topology. FIG. 1 is a simplified block diagram of the parallel-branch topology known in the prior art. Referring now to FIG. 1, parallel-branch predistortion circuits are characterized by a main path 100 for the electrical signal and a secondary path or paths 105, 110 for attenuators 112, the distortion generators 115, 120 and frequency-compensation adjustments 125, 130. A delay line 135 is normally included in the main path 100 for equalizing the delay between the main path 100 and the secondary paths 105,110.
Most parallel-branch second-order predistortion circuits are two-port circuits that generate second-order distortion and essentially no even-order distortion. Examples of parallel-branch predistortion circuits known in the art are shown in U. S. Pat. Nos. 5,361,156, 5,436,749, 5,481,389, 4,992,754, 5,132,639, 5,424,680, 5,418,637, 5,321,710, 5,243,613 and 5,252,930.
For the inline predistortion topology, the nonlinear elements are collectively considered to be one-port devices in series or in shunt with the primary path of the electrical signal. Both shunt and series elements may be used simultaneously in the primary path. The inline predistortion topology may have some advantages relative to the parallel-branch type predistortion circuits in simplicity and lower economic cost.
Patents and publications have described various implementations of one-port inline predistortion circuits. For example, U.S. Pat. No. 5,172,068 entitled "Third-Order Predistortion Linearization Circuit" issued on Dec. 15, 1992 to R. B. Childs discloses an inline predistorter circuit that cancels third-order distortion products produced by a nonlinear device, such as an optical transmitter. Additionally, U.S. Pat. No. 5,282,072 entitled "Shunt-Expansive Predistortion Linearizers for Optical Analog Transmitters" issued on Jan. 25, 1994 to M. Nazarathy, A. J. Ley, and H. C. Verhoeven discloses an inline predistorter circuit that suppresses third-order distortion products generated by another nonlinear device through the use of symmetrically configured diodes placed and biased back-to-back. Additionally, U.S. Pat. No. 5,703,530 entitled "Radio Frequency Amplifier Having Improved CTB and Cross Modulation Characteristics" issued on Dec. 30, 1997 to Sato, Yuzo, Kaneko, Katsumi, Saito, and Yasushi discloses inline predistortion circuits for canceling frequency dependent third-order distortions.
As described above, most one-port inline predistorter circuits known in the art relate primarily to third-order compensation of externally modulated optical transmitters. An inline second-order predistortion circuit is disclosed in U.S. Pat. No. 5,119,392 entitled "Second-Order Predistortion Circuit for Use with Laser Diode" issued Jun. 2, 1992 to Richard B. Childs. However, in this disclosure, distortion is generated in a two-port device and there are no means for eliminating unwanted third-order distortion from the distortion generator. Furthermore, U.S. Pat. No. 5,798,854 entitled "Inline Predistorters for Linearization of Electronic and Optical Signals" issued on Aug. 25, 1998 to H. A. Blauvelt and M. Regehr describes one-port inline predistorter circuits that compensate for both second-order and third-order distortion using a real distorter and an imaginary distorter.
In summary, there is a need for an equivalent one-port inline predistorter capable of substantially canceling second-order distortion generated by a nonlinear device, such as a distributed-feedback laser in a CATV system, but does not generate third-order distortion.